1. Field of the Invention
The present invention relates to devices for measuring lengths of sheets while the sheets are carried, and relates to image forming apparatuses such as copying machines and printers including the measuring devices.
2. Description of the Related Art
According to known methods for forming images on both sides of sheets in image forming apparatuses, toner images are first formed and fixed on first surfaces of the sheets. The sheets are then sent to double-sided printing paths so as to be reversed, and toner images are formed and fixed on second surfaces.
However, when the sheets are heated and pressurized during fixing of the toner images on the first surfaces, moisture content in the sheets is vaporized and fibers are extended, resulting in subtle changes in the size of the sheets after the fixation. Therefore, in order to adjust the positions of the images on the back surfaces of the sheets relative to those on the front surfaces, it is necessary for the image forming apparatuses to form images suitable for the image sizes on the second surfaces changed from those on the first surfaces after the passage of the sheets through fixing units.
In order to support the above-described phenomenon, an image forming apparatus that changes image sizes on second surfaces of sheets according to the change in the sheet size caused by the passage of the sheets through a fixing unit is disclosed in Japanese Patent Laid-Open No. 5-008460.
The image forming apparatus disclosed in the Japanese Patent Laid-Open No. 5-008460 defines variations in sheet size as a predetermined value, although the amount of vaporization of the moisture content in the sheets and the amount of elongation of the fibers are practically changed according to types of sheets and fixing conditions. That is, since the variations in sheet size are changed according to various conditions, the image forming apparatus needs to measure the actual sizes of the sheets so as to obtain more accurate variations.
An image forming apparatus measuring actual sizes of sheets is disclosed in Japanese Patent Laid-Open No. 3-172255. The image forming apparatus measures time from start of driving registration rollers to detection of the trailing end of a sheet using a sheet-detecting sensor, and converts the time into a length using carrying speed. Moreover, Japanese Patent Laid-Open No. 4-121345 (corresponding to U.S. Pat. No. 5,291,225) discloses an image forming apparatus that converts the number of clock pulses for driving a feed motor into a length by multiplying the clock pulses by the carrying speed of the sheet, the clock pulses being counted during a period between detection of the leading end and the trailing end of a sheet using a sheet-detecting sensor.
However, the carrying speed of the sheet is not simply determined by only the rotating speeds of the registration rollers or those of carrying rollers driven by the feed motor, and errors may be generated due to eccentricity or abrasion of the rollers. Therefore, when the rollers slip during carrying of a sheet, for example, the sheet size is not accurately detected in the technologies disclosed in Japanese Patent Laid-Open Nos. 3-172255 and 4-121345.